In rolling trains for plane products, such as strip, sheet and wide plate, the state of the art includes rolling stands, generally four-high, arranged in sequence, which progressively reduce the thickness of the product in transit.
In the roughing and pre-finishing passes, each rolling stand normally causes a reduction in thickness of a value between 30% and 50% compared with the thickness at inlet; the reduction limit is defined by the maximum value of the angle at which the rolled stock enters, the maximum rolling torque which can be applied and by the maximum rolling force.
The final thickness of the product is then defined either in a reversible finishing rolling mill for sheet or strip (for example of the steckel type), or in a finishing train with stands in tandem, wherein the percentages of reduction can generally be between 65% and 15% compared with the thickness at inlet.
In four-high rolling stands there are working rolls which act directly on the product to be rolled, and back-up rolls, of larger diameter and cooperating with a relative working roll, which have the function of supporting the rolling loads and, in particular, preventing flexions and deformations of the relative working rolls.
Motion is normally supplied to the working rolls of each rolling stand, particularly in finishing stands, but very often nowadays in roughing stands too, by means of transmission elements, known as spindles, which are moved by a single drive means though appropriate assemblies to reduce and double the motion.
Using a single drive means for both rolls theoretically ensures the transmission of an identical speed of rotation to the shafts of the rolls, so that, again theoretically, it reduces the possibility of an irregular and non-uniform drawing action of the rolled stock during the rolling pass.
However, in practice it has been found that, even when the speed of the spindles which transmit motion to the working rolls is constant and identical, as transmitted from the source of motion, the resistant torque of the two rolls is not the same, and this causes considerable irregularities in the rolling process, which negatively influence the functioning of the rolling stand.
On this point, please refer to the theoretical explanation given by Tselikov in "Stress and strain in metal rolling", Mir Publishers--Moscow 1967, chapter IV "Direction of the forces acting on the rolls during rolling", and particularly paragraphs 6, 7, 9 and 11.
According to measurements carried out on industrial plants, it has been found that a great difference between the torque transmitted by the upper spindle and the torque transmitted by the lower spindle, exceeding a standard ratio such as 40/60 (or 60/40), causes a tendency for the rolling process to become unstable, since horizontal thrusts occur on the rolled product.
These horizontal thrusts are due to the fact that, when there is a difference in the torque transmitted to the two working rolls, the rolling force which each roll transmits to the rolled stock tends to deviate from the vertical, generating respective horizontal components of an opposite direction and a variable intensity according to the entity of this difference.
The greater the horizontal forces, the more easily vibrations can occur, and these make the rolling force oscillate both in intensity and in direction.
If in addition to the dis-uniform torque we add the little irregularities in the transmission of motion between the two working rolls--irregularities caused by the mechanical parts which transmit motion from the motor to the rolls--vibrations, even strong vibrations, may be generated on the structure of the stand inasmuch as each roll tends to draw the rolled stock in a different way.
This makes the relative horizontal components of the rolling force dissimilar, and has repercussions on the rolls themselves and on the relative chocks.
Moreover, the rolled stock comes to be drawn irregularly and jerkily, which can cause damage and surface markings of the rolled stock and the rolling rolls.
There are very many factors which can cause different torque values transmitted to the two rolls.
A first cause is the different temperature of the two faces of the rolled stock and/or the different surface temperature of the working rolls.
A second cause is the different roughness of the working rolls.
Both the first and the second cause can be determined, for example, by the formation of pools of water on the upper face of the rolled stock due to inappropriate maintenance conditions.
Another cause is a different diameter of the working rolls, caused by different wear on one roll and the other or by grinding operations not carried out correctly.
A further cause is an inaccurate centering of the rolled stock with respect to the median plane of the rolls.
A further cause is a non-uniform metallic structure of the two faces of the rolled stock.
All these causes, and others, individually or combined, can cause great irregularities in the share of the torque to the rolls and, consequently, horizontal vibrations of the rolls; these vibrations make the rolling force imparted by the working rolls oscillate and thus generate irregular rolling.
These vibrations may also be caused by irregularities in the transmission of the motion which, in turn, cause torsional vibrations of the kinematic chain.
Vibrations are also caused in the bearings and the chocks of the working rolls.
The frequencies of vibration are generally syntonised with the 1st, 2nd or 3rd torsional frequency of the kinematic chain.
The state of the art also includes the use of systems to cool the rolls using fluids, particularly water, which is sprayed onto the surface of the rolls by appropriate collectors and nozzles. In order to have a more efficient heat exchange, normally there are greater deliveries of water in proximity with the area where the rolled product exits from the stand.
The spraying means comprise, or cooperate with, protection means which prevent the formation of pools of water on the upper face of the rolled stock passing through.
The cooling means have a part function of making uniform the surface temperature of the working rolls, but they have a very limited effect (which in any case cannot be controlled) on the other shortcomings which make the torque uneven and consequently generate vibrations in the stand.
The article "Compensation of a digitally . . . ", by Butler et al., taken from the journal "Institute of electrical and electronics engineers" vol. 1, n. Meeting 25, Oct. 7, 1990, pages 583-588, describes various techniques to minimize the excitation of resonance frequencies which lead to torsional vibration of the kinematic chain which transmits motion to the rolls of a rolling stand.
One of these techniques provides to vary the lubrication and surface finishing of the rolls.
This document teaches an empirical method which provides to adopt a posteriori corrections and strategies, after having detected the presence of torsional vibrations, to reduce or cancel said vibrations.
In other words, if the worker becomes aware that there are vibrations present, he activates or modifies the lubrication conditions, according to his knowledge or by empirical means, to modify the friction between the strip being rolled and the working rolls.
The document therefore does not teach any connection between the dis-uniform torque transmitted by the spindles to the working rolls and the lubrication conditions in order to reduce or cancel the vibrations and oscillations in the rolling force.
BE-A-890.928 also provides to act on the lubrication of the rolls to reduce the vibrations, but does not provide any indication concerning a measurement of the dis-uniform torque transmitted to the rolls as a basic element to establish the correction of the lubrication conditions.
Both these prior art documents describe an empirical method, which can be based only on successive and approximate adjustments; therefore they do not provide any guarantee either that the method will function efficiently and rapidly, or that optimum working conditions can be maintained for an acceptable period of time.
The present Applicant has devised and embodied this invention to overcome these shortcomings which businessmen in this field have long complained of, and to obtain further advantages as will be shown hereafter.